D. Kolar

On activated sintering and electrical properties of NaKNbO3

Abstract Sintering of Na 0.5 K 0.5 NbO 3 may be activated by slight excess of Nb 5+ ions or by incorporation of Mg 2+ ions into crystal lattice. By this means, high density samples (97% TD) were prepared by conventional sintering technique. Mg-doped sintered samples exhibit increase in k p , some lowering in e and large increase in resistivity after prolonged sintering.

Synthesis and crystal chemistry of BaNd2Ti3O10, BaNd2Ti5O14, and Nd4Ti9O24

Two new ternary compounds BaNd2Ti3O10 (1:1:3) and BaNd2Ti5O14 (1:1:5) have been identified in the BaONd2O3TiO2 system. Single crystals of the compounds were grown and unit cell dimensions and space group symmetry were determined. BaNd2Ti3O10 is orthorhombic with a = 3.8655 ± 0.0003, b = 28.156 ± 0.003 and c = 7.6221 ± 0.0007 A and possible space groups are Cmcm or Cmc2. The compound melts congruently at 1640 ± 20°C. BaNd2Ti5O14 is also orthorhombic with a = 22.346 ± 0.002, b = 12.201 ± 0.001 and c = 3.8404 ± 0.0003 A and possible space groups are Pbam and Pba2. This compound melts congruently at 1540 ± 20°C. Single crystals of the binary compound Nd4Ti9O24 were also grown and found to be orthorhombic with a = 35.289 ± 0.003, b = 13.991 ± 0.001, c = 14.479 ± 0.001 A, space group Fddd.

Electrical and microstructural characterisation of (La0.8Sr0.2)(Fe1 − xAlx)O3 and (La0.8Sr0.2)(Mn1 − xAlx)O3 as possible SOFC cathode materials

The perovskites with nominal compositions (La0.8Sr0.2)(Fe1 − xAlx)O3 and (La0.8Sr0.2)(Mn1 − xAlx)O3 (x from 0 to 0.94) were evaluated as possible solid oxide fuel cell (SOFC) cathodes. Cell parameters of solid solutions were calculated. The electrical and microstructural characteristics and high temperature interactions with YSZ were studied. As compared with ‘pure’ perovskites, doping with strontium and aluminium decreases and increases their specific resistivity, respectively. The incorporation of alumina and strontium oxide substantially reduces the sinterability resulting in a rather porous, fine grained microstructure. The reaction rate between perovskite materials and YSZ at high temperatures is higher for lanthanum manganites than for lanthanum ferrites, and the partial exchange of cations on ‘B’ sites with aluminium decreases the reaction rate.

Role of Bi 2 O 3 in optimizing the dielectric properties of Ba 4.5 Nd 9 Ti 18 O 54 based microwave ceramics

Small additions of Bi{sub 2}O{sub 3} or bismuth titanate improve the dielectric properties of Ba{sub 4.5}Nd{sub 9}Ti{sub 18}O{sub 54} at microwave frequencies. It was found that Bi{sup 3+} can replace Nd{sup 3+} up to 15 mol {percent}, the limiting composition of solid solution being Ba{sub 4.5}(Nd{sub 0.85}Bi{sub 0.15}){sub 9}Ti{sub 18}O{sub 54}. At the solid solution limit the temperature coefficient of resonant frequency attains its minimum value. The modified ceramic is distinguished by high permittivity of 99 and a {ital Q}-value of 5500. The temperature coefficient of resonant frequency is low, 15 ppm/K. After exceeding the solid solubility limit, additional Bi{sub 2}O{sub 3} concentrates as a Bi-rich phase at the grain boundaries, causing considerable reduction of the {ital Q}-value and an increase of {tau}{sub {ital f}}. {copyright} {ital 1996 Materials Research Society.}

Interactions between a thick film LaMnO3 cathode and YSZ SOFC electrolyte during high temperature ageing

Abstract Reactions between thick film LaMnO 3 cathodes and YSZ substrates were investigated by ageing at 1450 °C. Also, subsolidus phase equilibria in the La 2 O 3 Mn 2 O 3 ZrO 2 system were confirmed by X-ray powder diffraction analysis. A La 2 Zr 2 O 7 phase formed on the YSZ/LaMnO 3 interface. The Mn 2 O 3 released in the reaction partly diffused in to YSZ and partly evaporated. Diffusion of Y and Zr into LaMnO 3 was not detected. After a prolonged period of ageing (100 h) the cathode layer is separated along most of the YSZ/LaMnO 3 interface with only a few sintered contacts. On the surface of large pores where LaMnO 3 separated from the YSZ substrate “islands” of La silicate phase was found. Silica originated from grain boundaries in YSZ. The presence of La silicate phase could be a reason for separation of the LaMnO 3 layer from YSZ substrate after prolonged high-temperature ageing.

Phase stability and equilibria in the La2O3–TiO2 system

Abstract XRD, electron probe wavelength and energy dispersive X-ray analyses were used to reexamine the phase relations in the La 2 O 3 –TiO 2 system. The diagram was redrawn to include the compound La 4 Ti 3 O 12 in addition to La 4 Ti 9 O 24 , La 2 Ti 2 O 7 and La 2 TiO 5 . Above 1455°C a cation deficient perovskite La 2/3 TiO 3 is stabilized by a small number of Ti 3+ ions and remains stable on cooling in air. The proposed diagram represents a section through the system at the normal oxygen pressure in air at 1 atm, compositions being expressed in terms of the oxide components stable at room temperature.

Microwave dielectric properties of ceramics with compositions along the La2/3TiO3(STAB)–LaAlO3 tie line

The stabilization of La2/3TiO3 in the perovskite structure by 4 mol% LaAlO3 resulted in maximum values for the relative permittivity (k′≈72) and temperature coefficient of resonant frequency (τf=123 ppm K-1). The Q-value quality factor of these ceramics were over 5300 measured at 4.5 GHz. Higher contents of LaAlO3 cause a decrease in both the permittivity and τf values, accompanied by an increase in the Q-value. Typical values measured at 6.0 GHz for ceramics with the composition 70 mol% La2/3TiO3 – 30 mol% LaAlO3 are k′>44, τf=+7 ppm K-1 and Q>5500. The results depend on the processing parameters. The sintering temperature increases with increasing LaAlO3 content. An oxidizing sintering atmosphere improves the Q-value, whereas the permittivity and τf are not significantly affected.

Low-temperature synthesis of sinterable SiC powders by carbothermic reduction of colloidal SiO2

Abstractβ-SiC powders were synthesised by carbothermic reduction of carbon-black doped silica gel. The morphological characteristics of the resulting powders depend on the morphology of precursor carbon black as well as the reaction conditions. A model for particle coarsening during carbothermic reduction is presented, suggesting that surface diffusion may be responsible for the formation of polycrystalline aggregates, while at higher reaction temperatures volume diffusion is operative. Processing consisting of decarburization, HF treatment and sedimentation but no milling is required to convert crude reaction products into sinterable powders. After doping with boron and carbon they can be sintered to 98–99% of theoretical density which, in general, is better than or comparable to results obtained with commercial β-SiC powders which were also included in the present study.

The phase diagram of CaTiO3-SrTiO3

Abstract Phase equilibria in the binary system CaTiO 3 -SrTiO 3 were studied above 1300°C. This system is characterized by a complete series of solid solutions, with a minimum in the solidus at 23 mol% SrTiO 3 and 1940°C. The volume change of the unit cell of CaTiO 3 as a function of composition shows very little deviation from Vegards law for solid solutions. No immiscibility gap was found.

The role of dopants in tailoring the microwave properties of Ba6-xR8+2/3x Ti18O54 R = (La–Gd) Ceramics

Microwave dielectric ceramics with a high dielectric constant need to satisfy very high technical demands. They should possess extremely low losses to achieve high Q-values (Quality factor) a small temperature coefficient of resonant frequency (τf), and a relative permittivity (εr) higher than 80. Industrial applications require very stringent electrical and dimensional tolerances, typically ± 0.5–1.0 ppm K-1 for a specified τf and ± 0.25% for a specified εr. To meet such requirements ceramics based on BaO–R2O3 – TiO2 (R = La–Gd) are used. The investigation of this type of ceramic was stimulated by the observation that ceramics based on compositions in the TiO2-rich region of the system exhibit highly temperature stable electrical properties. Especially interesting are compositions within the solid solubility region with the general formula Ba6-xR8+2/3x Ti18O54. As the ionic radius of the rare earth decreases the extent of the solid solubility region becomes narrower, i.e., 0<x<3 for La and x = 0.5 for Gd. Further improvements in the dielectric microwave properties can be achieved by combining different rare earth oxides, and by partial replacement of Ba2+ with other alkaline earth atoms such as Ca2+ and Sr2+. Typically such ceramics meet the requirements for Q and εr; however, τf must be additionally adjusted by the use of dopants. Most commonly bismuth and lead oxides or titanates are used. In the present contribution the role of different dopants and their influence on the resulting microwave dielectric properties of Ba6-xR8+2/3x Ti18O54 based ceramics are discussed.